1. Field of the Invention
The present invention relates to a method, optical device, and system for optical fiber transmission.
2. Description of the Related Art
Owing to recent developments of low-loss silica optical fibers, various optical fiber communication systems each using such an optical fiber as a transmission line have been put to practical use. The optical fiber itself has a very wide band. However, a transmission capacity by the optical fiber is actually limited by a system design. The most important limitation is due to waveform distortion by chromatic dispersion occurring in the optical fiber. Further, the optical fiber attenuates an optical signal at a rate of about 0.2 dB/km, for example. Loss of the optical signal due to this attenuation has been compensated for by adopting an optical amplifier such as an erbium doped fiber amplifier (EDFA) that is a typical example.
The chromatic dispersion that is often referred to simply as dispersion is a phenomenon such that the group velocity of an optical signal in an optical fiber changes as a function of the wavelength (frequency) of the optical signal. In a standard single-mode fiber, for example, an optical signal having a longer wavelength propagates faster than an optical signal having a shorter wavelength in a wavelength region shorter than 1.3 μm, and the resultant dispersion is usually referred to as normal dispersion. In contrast, an optical signal having a shorter wavelength propagates faster than an optical signal having a longer wavelength in a wavelength region longer than 1.3 μm, and the resultant dispersion is usually referred to as anomalous dispersion.
In recent years, the nonlinearities of an optical fiber have received attention in association with an increase in optical signal power due to the use of an EDFA. The most important nonlinearity that limits a transmission capacity is an optical Kerr effect occurring in an optical fiber. The optical Kerr effect is a phenomenon such that the refractive index of an optical fiber changes with the intensity of an optical signal. A change in the refractive index modulates the phase of an optical signal propagating in an optical fiber, resulting in the occurrence of frequency chirping which changes a signal spectrum. This phenomenon is known as self-phase modulation (SPM). Spectral broadening due to SPM occurs to cause further enlargement of the waveform distortion due to chromatic dispersion.
Thus, in optical fiber communication, the waveform degradation due to the dispersion of an optical fiber or the nonlinear optical effects in an optical fiber becomes a large factor of transmission limit. The influence of the dispersion increases with an increase in width of a signal band, and becomes significant in proportion to the square of a signal speed. Accordingly, a transmission distance in transmission of a high-speed signal is remarkably limited. Various dispersion compensating methods have been invented and put to practical use to compensate for the dispersion.
As a typical example of the methods for compensating the transmission waveform distortion due to dispersion, a method using a dispersion compensator is known. A dispersion compensating fiber providing a large dispersion or a compensating device such as a fiber grating is known as the dispersion compensator. Other known dispersion compensating methods include a method of alternately arranging positive and negative dispersions along a transmission line to configure a transmission line having substantially zero dispersion, and a method of arranging an optical phase conjugator along a transmission line to compensate for a phase change due to dispersion. The compensating method using the optical phase conjugator can also compensate for the nonlinear optical effects.
As an example of the method of simply compensating the influence of the nonlinear optical effects, a method of performing prechirping to a signal is known. This method is a method of compressing pulses by the nonlinear optical effects in a transmission line to compensate for pulse broadening due to dispersion and to simultaneously ensure a high optical signal-to-noise ratio (SNR), thereby increasing a transmission distance. This prechirping method is widely adopted in a practical system.
The group velocity of optical pulses in a normal dispersive fiber becomes larger with an increase in wavelength, whereas the group velocity of optical pulses in an anomalous dispersive fiber becomes larger with a decrease in wavelength. Accordingly, by providing chirping such that a wavelength shift toward longer wavelengths occurs near the leading edge of each pulse (negative chirp) and a wavelength shift toward shorter wavelengths occurs near the trailing edge of each pulse (positive chirp), i.e., by providing up-chirp, pulse compression is caused by transmission in an anomalous dispersive fiber. On the other hand, by providing chirping such that a wavelength shift toward shorter wavelengths occurs near the leading edge of each pulse (positive chirp) and a wavelength shift toward longer wavelengths occurs near the trailing edge of each pulse (negative chirp), i.e., by providing down-chirp, pulse compression is caused by transmission in a normal dispersive fiber.
Further, pulse compression can also be caused by setting the total dispersion to a slight normal dispersion or a slight anomalous dispersion rather than to zero dispersion in carrying out dispersion compensation, and then performing the prechirping under this setting.
In the conventional dispersion compensating methods or the prechirping method, the transmission distance cannot be sufficiently increased.
It is therefore an object of the present invention to provide a method for optical fiber transmission which can increase a transmission distance. It is another object of the present invention to provide an optical device and system which are applicable in carrying out such a method. Other objects of the present invention will become apparent from the following description.